The present invention concerns a method for lubricating and/or cooling a transmission unit in motor vehicles, as well as a transmission unit applying this method.
In particular, it concerns transmission units of the type whereby the transmission ratio is regulated in a continuously variable way by means of a continuously variable transmission.
Such a continuously variable transmission includes of two pulleys in between which is provided an endless transmission element. The pulleys have conical pulley halves, of which one half can be axially moved, such that the running radiuses of the transmission element on the pulleys can be changed by moving the above-mentioned pulley halves.
The pulley halves are moved and/or pressed on by means of at least one cylinder/piston unit. In the pressure chamber of this cylinder/piston unit is provided a static pressure in order to press on and/or move the accompanying pulley half. At high rotational speeds, undesired centrifugal pressure is added on top of this static pressure.
In order to compensate for the centrifugal pressure, it is known to equip the above-mentioned transmission units with a pressure compensation chamber, which is connected to the above-mentioned pressure chamber via a passage, such that on either side of the piston of the cylinder/piston unit is built up a pressure.
The pressure compensation chamber connects with the environment via an opening, such that the superfluous hydraulic medium, usually oil, can flow away via this opening, which is then collected in the oil sump of the transmission unit.
It is also known to lubricate and/or cool the transmission element by spraying hydraulic medium on the endless transmission element.
Further, it is also known that the transmission ratio of the transmission unit is controlled as a function of a large number of factors, whereby a large number of controls are carried out by means of hydraulic valves. Both the cylinder/piston units and the hydraulic valves require a large liquid flow in a number of situations, such as in the case of creep, road-load change up and hard braking.
In order to guarantee the flow rate in such situations, a relatively large pump must be provided for the supply of the medium, as a result of which the efficiency of the transmission unit declines.
According to another possibility, a pump is used which is sufficiently large to guarantee the supply of hydraulic medium in any normal situation, and the least important consumers of the liquid supply are cut off in critical situations. Usually, these are the consumers which have a mere lubrication and/or cooling function.
On the one hand is obtained in this way that the transmission characteristics which are observed by the driver are not influenced in any way, but on the other hand there is a disadvantage in that a number of parts are loaded more heavily as they are cooled and/or lubricated less than normally.